This application is a 371 of PCT/EP99/00835 filed on Feb. 9, 1999.
The present invention relates to novel mono- and polymetallic magnetic colloid particles (e.g., Fe, Co, Ni, Fe/Co) of a size of up to 20 nm the surface of which is protected from corrosion by precious metals, e.g., Pd, Ag, Pt or Au, and a process for the preparation of these materials.
Various methods are known for the preparation of unprotected colloidal magnetic metals, especially Fe, Co and Ni, e.g., salt reduction (G. Schmid (Ed.), Clusters and Colloids, VCH, 1994, EP 423 627, DE 44 43 705 and U.S. Pat. No. 5,620,584), thermal, photochemical and sonochemical decomposition of metal carbonyls and nitrosyl complexes (K. S. Suslick, T. Hyeon, M. Fang, A. A. Cichowlas in: W. Moser (Ed.), Advances Catalysts and Nanostructured Materials, Chapter 8, p. 197, Academic Press, 1996), and the reduction of salts or the decomposition of carbonyl compounds in micellar solutions (O. A. Platonova, L. M. Bronstein, S. P. Solodovnikov, I. M. Yanovskaya, E. S. Obolonkova, P. M. Valetsky, E. Wenz, M. Antonietti, Colloid Polym. Sci. 275, 1997, 426). The long-term stability of such previously proposed colloidal magnetic metals against atmospheric oxygen is unsatisfactory, however (see Comparative Examples: Table 1, Nos. 2, 3 and 5, FIGS. 1a, 2 and 4).
Therefore, it has been the object of the present invention to provide a process for the preparation of corrosion-stable colloidal magnetic nanometals of a size of up to 20 nm by protecting the particle surface against corrosive attack by means of precious metal coatings.
Japanese Patent JP 0727 2922 AZ describes the preparation of anticorrosive, resin-bound Fe magnets protected by three coatings with, inter alia, precious metals. However, they are exclusively coated magnetic bulk materials which are not suitable for nanotechnology and magnetic fluids. A process for the preparation of precious-metal protected magnetic nanocolloid particles of a size of up to 20 nm has not been known. Toshima et al. describe the preparation of Pd-Pt bimetal colloids (1.5-5.5 nm) with a controllable core-shell structure (Y. Wang and N. Toshima, J. Phys. Chem. B, 1997, 101, 5301). Schmid et al. describe the preparation of gold-coated Pd particles of a size of from 20 to 56 nm having a layer structure (G. Schmid, H. West, J. -O. Maim, J. -O. Bovin, and C. Grenthe, Chem. Eur. J. 1996, 1099). However, the mentioned processes cannot be transferred to a combination of magnetic metal (Fe, Co, Ni) and precious metal coating. J. Sinzig tried to protect the particle surface of an N(octyl)4-stabilized Co colloid from corrosion by chemical plating with elemental gold (J. Sinzig, Proefschrift, p. 74, Rijksuniversiteit te Leiden (NL) 1997). The following redox process occurs at the Co surface: 12 Co(0)+2 AuCl3xe2x86x92Co9Au2+3 CoCl2. Although the oxidation stability of the materials can be enhanced in this way, it is still insufficient for the mentioned applications (see Comparative Example: Example No. 8, Table 1 No. 6, FIGS. 1b and 6).
It has now surprisingly been found that corrosion-stable magnetic nanocolloids can be obtained by preparing, e.g., Fe, Co, Ni or Fe/Co alloy colloids by methods known from the literature (see above) or generating them in situ, treating them, under extremely strict exclusion of atmospheric oxygen in organic solvents, with strong reductants, e.g., hydrides of elements from main groups 1 to 3 of the Periodic Table, complex hydrides of these elements or of tetraalkylammonium, or reducing organometallic compounds of main groups 1 to 4 of the Periodic Table, and adding precious metal salts, e.g., of Pd, Ag, Pt or Au, preferably in solution in a molar ratio (Colloid:precious metal salt) of  greater than 1:1, preferably 1:0.3, to the resulting mixture. Suitable solvents include aliphatic and aromatic solvents and ethers, and suitable reductants include, e.g., the above mentioned hydrides and organometallic compounds in a molar ratio (reductant:colloid) of at least 1:1, preferably  greater than 3:1.
The thus obtained precious-metal protected anticorrosive magnetic nanocolloids of a size of up to 20 nm have long-term stability; for example, in the Au-protected Fe colloid, a decrease of magnetization J by corrosion cannot be detected until the measurement is terminated after 100 hours. The materials can be employed in isolated form or in solution, without intending to limit their use, e.g., as a sealing medium against dust and gases in magnetic fluid seals (liquid O ring), for the lubrication and bearing of rotating shafts (magnetic levitation bearing), for magnetooptical storage of information, e.g., in compact disks and minidisks, and further, after applying an additional cell-compatible coating, for the magnetic labeling of cells and their magnetic separation in biological samples, or for the topical application of medicaments. The superior corrosion stability of the new materials as compared to unprotected magnetic nanocolloids of similar size will be illustrated by the following Examples (Examples 1 to 7, Table 2, FIGS. 1a, 1b, 3 and 5).